The present invention relates to a single gate-type nonvolatile semiconductor memory device including a charge storage layer formed of a stacked film of silicon oxide film, silicon nitride film and silicon oxide film, and a method for fabricating the same.
As rewritable nonvolatile semiconductor memory devices, semiconductor memory devices, such as EEPROMs, flash EEPROMs, etc., which store information by storing charges in floating gates, are generally known. Such semiconductor memory devices require floating gates for storing information, in addition to control gates which function as word lines, and accordingly two conductor layers are required to form the memory cell transistors.
On the other hand, as a nonvolatile semiconductor memory device which has a simpler structure and is easy to be highly integrated, a nonvolatile semiconductor memory device including the memory cell transistors having single gates is proposed.
A conventional nonvolatile semiconductor device including the single gates will be explained with reference to FIG. 12. FIG. 12 is a sectional view of the conventional nonvolatile semiconductor memory device.
A plurality of bit lines 114 of an n+ diffused layer are formed on a silicon substrate 100, extended normally to the sheet of the drawing. A bit line oxide film 116 is formed on the bit lines 114. A pocket layer 112 of a pxe2x88x92 diffused layer is formed on both sides of the silicon substrate 100 in the regions between the bit lines 114. Charge storage layer 108 of a stacked film of a silicon oxide film 102, a silicon nitride film 104 and a silicon oxide film 106 is formed on the silicon substrate 100 between the bit lines 114. A plurality of word lines 124 are formed on the bit line oxide film 116 and the charge storage layer 108, extended crossing the bit lines 114. Thus, the single gate-type memory cell transistors having control gates formed of the word lines 124 are formed.
Then, the method for fabricating the conventional nonvolatile semiconductor memory device shown in FIG. 12 will be explained with reference to FIGS. 13A-13D and 14A-14D. FIGS. 13A-13D and 14A-14D are sectional views of the conventional nonvolatile memory device in the steps of the method for fabricating the same, which show the method.
First, an about 200-800 nm-thick device isolation film (not shown) is formed on the silicon substrate 100 by, e.g., the usual LOCOS method to define device regions. The device isolation film is not formed in the memory cell regions.
Next, an about 5-10 nm-thick silicon oxide film 102 is formed on the silicon substrate 100 with the device isolation film formed on by, e.g., thermal oxidation method or CVD method.
Then, an about 2-15 nm-thick silicon nitride film 104 is formed on the silicon oxide film 102 by, e.g., CVD method.
Next, an about 5-10 nm-thick silicon oxide film 106 is formed on the silicon nitride film 104 by, e.g., CVD method.
Thus, the charge storage layer 108 of a stacked structure of the silicon oxide film 102, the silicon nitride film 104 and the silicon oxide film 106 is formed (FIG. 13A).
Then, a photoresist film 110 for exposing regions for the bit lines 114 to be formed in is formed on the charge storage film 108 by the usual lithography. The photoresist film 110 has a stripe pattern extended normally to the drawing sheet.
Then, with the photoresist film 110 as a mask, B+ (boron) ions are implanted to form in the silicon substrate 100 the pxe2x88x92 diffused layer 112 which is to be the pocket layer (FIG. 13B). The B+ ions are implanted, for example, at about 20-40xc2x0 to a normal to the silicon substrate 100, at acceleration energy of 50-60 keV, and at a dose of 1.0-3.0xc3x971013 cmxe2x88x922.
Next, the silicon oxide film 106 and the silicon nitride film 104 are etched by dry etching with the photoresist film 110 as a mask (FIG. 13C).
Then, As+ (arsenic) ions are implanted with the photoresist film 110 as a mask to form in the silicon substrate 100 the bit lines 114 which function also as the source/drain diffused layer regions (FIG. 13D). As+ ions are implanted, for example, at acceleration energy of 50-60 keV and at a dose of 1.0-3.0xc3x971015 cmxe2x88x923. In the previous step the silicon oxide film 102 is not removed, and remains. This is for the prevention of contamination of the silicon substrate 100 in this ion implanting step.
Next, the photoresist film 110 is removed by the usual ashing.
Then, the silicon substrate 100 is thermally oxidized to form the bit line oxide film 116 of about 50-100 nm-thick on the bit lines 114. The silicon substrate 100 in the regions between the bit lines 114 is not oxidized because of the silicon nitride film 104 which functions as an oxidation mask.
Then, an about 5-10 nm-thick silicon oxide film 118 is formed on the silicon substrate 100 by thermal oxidation (FIG. 14A). The silicon oxide film 118 is a coating film for preventing the silicon nitride film 104 from exposing to thereby deteriorate data retention characteristics.
Next, a conductor film which is to be the word lines is deposited on the entire surface. For example, first an about 100-150 nm-thick polycrystalline silicon film 120 is deposited. Then, P (phosphorus) as an impurity is heavily introduced into the polycrystalline silicon film 120 by, e.g., vapor phase diffusion or ion implantation to make the polycrystalline silicon film 120 less electrical resistance. Then, an about 100-150 nm-thick WSi (tungsten silicide) film 122 is deposited on the polycrystalline silicon film 120 by, e.g., CVD method. Thus, the polycide structure of the stacked film of the WSi film 122 and the polycrystalline silicon film 120 is formed.
Next, the stacked film of the WSi film 122 and the polycrystalline silicon film 120 is patterned by the usual lithography and etching to form the word lines 124 of the stacked film of the WSi film 122 and the polycrystalline silicon film 120. A plurality of the word lines 124 are extended, crossing the bit lines 114.
Then, ion implantation is performed with the bit lien oxide film 116 and the word lines 124 as a mask to form a channel cut layer (not shown) for the isolation of the memory cells. The channel cut layer is formed by implanting B+ ions, for example, at 20-30 keV acceleration energy and at a dose of 1.0-3.0xc3x971012 cmxe2x88x922.
Next, an about 20-30 nm-thick silicon nitride film, an about 100-150 nm-thick silicon oxide film and an about 600-900 nm-thick BPSG film, etc. are sequentially deposited on the entire surface by, e.g., CVD method to form an inter-layer insulation film 126 of the stacked film of these insulation films.
Then, a required interconnection layer, etc. are formed on the inter-layer insulation film 126 by the usual semiconductor fabrication method.
Thus, the nonvolatile semiconductor memory device including the single gates is fabricated.
However, in the above-described conventional nonvolatile semiconductor memory device, as shown in FIG. 15A, the ion implantation for forming the pxe2x88x92 diffused layer 112 is performed after the charge storage layer 108 has been formed, which often damages the charge storage layer 108 near the drain regions and the charge storage layer 108 near the source regions (see FIG. 15B). The charge storage layer 108 especially near the source/drain regions (the actual charge storage layer is the silicon nitride film 104) is a region where electrons are captured to retain information. The damage of the region deteriorates charge retention characteristics, which often leads to deterioration of cycling characteristics and data retention characteristics of the nonvolatile semiconductor memory device.
In the conventional nonvolatile semiconductor memory device, as shown in FIG. 16A, the silicon oxide film 106 and the silicon nitride film 104 are etched with the silicon oxide film 102 as a stopper. The silicon nitride film 102 is so thin that the silicon oxide films 102, 104 are often side-etched due to shortage of a selective ratio between the silicon nitride film and the silicon oxide film, or even the base silicon oxide film 102 is often etched (see FIG. 16B). Etching damage often extends even to the charge storage layer 108 and the inside of the silicon substrate 100, which deteriorates device characteristics.
In fabricating a semiconductor device including nonvolatile semiconductor memory elements and logic elements, it is important to depress addition to a number of fabrication steps and fabricate the semiconductor device without sacrificing high-speed operation of the logic unit.
An object of the present invention is to provide a method for fabricating a nonvolatile semiconductor memory device which can restrain the damages of the charge storage layer and the base substrate so as to improve cycling characteristics and data retention characteristics of the nonvolatile semiconductor memory device.
Another object of the present invention is to provide a method for fabricating a nonvolatile semiconductor memory device which can be easily rationalized to be together with a logic device fabricating method.
The above-described objects of the present invention are achieved by a method for fabricating a nonvolatile semiconductor memory device comprising the steps of: forming a insulation film on a semiconductor substrate of a first conduction type; introducing an impurity into the semiconductor substrate through the insulation film to form a source diffused layer and a drain diffused layer of a second conduction type which is different from the first conduction type, and a pocket layer of the first conduction type adjacent to the source diffused layer and the drain diffused layer; removing the insulation film; forming a charge storage layer on the semiconductor substrate; and forming a gate electrode on the charge storage layer between the source diffused layer and the drain diffused layer.
In the above-described method for fabricating a nonvolatile semiconductor memory device, it is possible that the insulation film includes at least one silicon nitride film; and the method further comprises, after the step of forming the source diffused layer, the drain diffused layer, and the pocket layer, the step of selectively oxidizing the semiconductor substrate with the silicon nitride film as a mask to form a bit line oxide film.
In the above-described method for fabricating a nonvolatile semiconductor memory device, it is possible that the method further comprises, before the step of forming the source diffused layer, the drain diffused layer, and the pocket layer, the step of selectively oxidizing the semiconductor substrate with the silicon nitride film as a mask to form a device isolation film.
In the above-described method for fabricating a nonvolatile semiconductor memory device, it is possible that the method further comprises, before the step of forming the insulation film, the step of forming a device isolation film, and in which the insulation film is a sacrificial oxidation film which is formed after forming the device isolation film and is removed before forming the charge storage layer.
In the above-described method for fabricating a nonvolatile semiconductor memory device, it is possible that the charge storage layer includes at least one silicon nitride film, and the method further comprises after the step of forming the charge storage layer, the step of selectively oxidizing the semiconductor substrate with the silicon nitride film as a mask to form a bit line oxide film.
In the above-described method for fabricating a nonvolatile semiconductor memory device, it is possible that the charge storage layer includes at least one silicon oxide film, and the silicon oxide film is formed on the semiconductor substrate by thermal oxidation so as to have a large film thickness on the source diffused layer and the drain diffused layer than a film thickness on a rest region.
According to the present invention, the pxe2x88x92 diffused layer forming the pocket layer, and the bit lines are formed without performing the ion-implantation through the charge storage layer, whereby damage due to the ion implantation is not introduced into the charge storage layer as in the conventional nonvolatile semiconductor memory device fabrication method, in which the ion implantation is performed through the charge storage layer. Thus, deterioration of cycling characteristics and data retention characteristics of the nonvolatile semiconductor memory device can be prevented.
Because of the thick oxide film formed below, damage made to the base substrate can be restrained in the step of patterning the charge storage layer. Accordingly, the method for fabricating the nonvolatile semiconductor memory device according to the present invention can restrain deterioration of device characteristics due to the etching damage in comparison with the conventional nonvolatile semiconductor memory device.
The through oxide film for forming the diffused layers of the nonvolatile semiconductor memory device is used also as an oxide film used in forming a logic device, and the pad oxide film and the silicon nitride film as the oxidation mask used in forming the device isolation film are used also in the step of forming the nonvolatile semiconductor memory device, whereby the fabrication steps can be rationalized. Accordingly, in forming a semiconductor device with the nonvolatile semiconductor memory device and a logic device mounted together, the semiconductor device can be fabricated without adding a large number of fabrication steps.